Opioids such as morphine are highly prescribed for moderate to severe pain, however, their euphoric properties contribute to their widespread abuse. The [unreadable]-opioid receptor is primarily responsible for both the analgesic and euphoric effects, while agonists at the d -opioid receptor retain some analgesic properties, but have a lower abuse liability and have been suggested to attenuate some of the negative effects of [unreadable] agonists. For instance, eliminating the action of the d -receptor blocks the development of morphine tolerance and dependence in rodents. This interaction could be explained by heterodimerization of [unreadable] and d receptors, or by compartmentalization into membrane microdomains, such as lipid rafts, to form functional signaling complexes. In support of this, [unreadable] and d receptors colocalize in certain brain neurons and can heterodimerize in biochemical assays, while n and K receptors localize to lipid rafts with functional signaling consequences. The hypothesis of this proposal is that lipid rafts are required for [unreadable] and d -opioid receptor pharmacology including cross-talk. Two specific aims have been designed to test this hypothesis. The goal of specific aim #1 is to investigate the role of lipid rafts on [unreadable] and d receptor signaling by addressing the following questions: a) Are [unreadable] and d receptors and their associated signaling proteins localized to lipid rafts? and b) Are lipid rafts required for efficient signaling of [unreadable] and d receptors and for cross-talk between these receptors? All experiments in aim 1 will be performed in HEK293 cells expressing epitope-tagged [unreadable] and/or d receptors. The goal of specific aim #2 is to determine the impact of cholesterol-lowering on [unreadable]/ d receptor interactions in vivo, by measuring [unreadable] and d agoinst-mediated antinociception, tolerance and dependence in mice treated with the clinically-used cholesterol-lowering agent simvastatin. The effect of cholesterol-lowering on opioid signaling will provide insight into the mechanism behind the cross-talk between [unreadable] and d receptors. This understanding could lead to the development of highly efficacious analgesics with a decreased abuse liability. Generally, the effect of cholesterol-lowering on opioid and GPCR signaling is important to study because of the increasing use of cholesterol-lowering agents and the continuously lowering clinical cholesterol goals.